You're better off checking e-bay or perhaps the buy/sell section of this Forum. I know of a few sites that sell the OEM replacement case, but I do not see any selling the backdoor battery cover, just the front face plates with lens.
Good luck on your hunt. Sucks to hear what happened. Invest in a case or keep it in a holster on you. Things could have been worst, you could have lost the phone all together.